This invention relates to a PWM inverter apparatus, and more particularly to a gate signal generating method and device suitable for controlling a rotating magnetic flux in an a.c. motor driven by a PWM inverter.
A manner of control is called PWM control when a three-phase inverter driving an a.c. motor is controlled by a pulse-width modulated (PWM) gate signal. In the case of the so-called PWM control, it is known that such a PWM gate signal is generated on the basis of a detected rotating magnetic flux in the a.c. motor which is the load of the three-phase inverter or on the basis of an externally-applied rotating magnetic flux command.
JP-A-No. 59-25592 describes a method of controlling an output voltage vector of an inverter driving a load such as an induction motor by detecting a rotating magnetic flux in the induction motor, comparing this detected rotating magnetic flux with a predetermined reference rotating magnetic flux and, when the former deviates from the latter, correcting the error to zero.
JP-A-Nos. 61-227696 and 62-16092 describe a method of controlling an output voltage vector of an inverter by setting a vector of a virtual rotating magnetic flux related to that of a reference rotating magnetic flux, comparing these vectors and, when the former deviates from the latter, correcting the error to zero.
The control of the inverter output voltage vector is also described in JP-A-58-39278 corresponding to U.S. Pat. No. 4,488,215 in addition to JP-A-61-22769 and JP-A-62-16092 cited above. According to the disclosures of these publications, six voltage unit vectors v and two zero vectors v.sub.0 determined by combination of on-off states of the inverter's switching elements of three-phase bridge arrangement are suitably combined to control the output voltage vector of the inverter.
However, in the case of the prior art method and apparatus for controlling the inverter output voltage vector on the basis of the deviation of the detected rotating magnetic flux from the reference rotating magnetic flux, a magnetic flux detector for detecting the rotating magnetic flux is necessarily required, resulting in a complicated structure of the apparatus.
Also, in the case of the prior art method and apparatus for controlling the inverter output voltage vector on the basis of the deviation of the detected rotating magnetic flux from the reference rotating magnetic flux, as well as the prior art method and apparatus for controlling the inverter output voltage vector on the basis of the deviation of the virtual rotating magnetic flux vector from the reference rotating magnetic flux vector, the sequence of processing from the step of data sampling for detecting the deviation to the step of controlling the voltage vector must be carried out at the same time, that is, the sequence of processing must be substantially instantaneously executed so as to prevent degradation of the accuracy of control. Therefore, high-speed processing means is necessarily required in order to follow up the rotating magnetic flux command to attain the desired control with high accuracy. In other words, there arises the problem that the accuracy of control is inevitably restricted by the speed of processing. Another problem is that the PWM switching frequency is limited by the processing period described above and cannot be increased.
Further, because the voltage vectors are selected so as to confine all the instantaneous errors within an allowable range, and the switching elements of the inverter are turned on-off on the basis of the selected voltage vectors, the order of turning on-off the switching elements of the individual phases, that is, the switching mode is irregularly changed. Therefore, there arises the problem that the switching frequency of the inverter's switching elements per unit time cannot be predicted for the control purpose. This problem exerts a great influence on the selection of the switching tolerance determined in relation to the heat tolerance of the switching elements of the inverter.